Scaling objects without scaling children
There are many ways to do what you've described. It's kind of difficult to know which method will suit your particular project. But I will offer some ideas.
Method 1.
Parent each object to the bones of a two-bone Armature. Then set the child bone to not inherit scale.
Method 2.
Use Vertex Parenting to parent the upper object to the vertices of the top part of the lower object.
Method 3.
Join them as a single object, then create a Shape Key wherein the bottom part is taller but the top part is simply raised (not scaled).
Method 4.
Just move them (super-basic).
If this doesn't need to animate, then it's as simple as grabbing the upper object, translating it up on the Z axis by some value (you can type it in), and then going into Edit Mode and doing the same for just the vertices on the upper part of the lower object.
Vertex Snapping is also a handy tool for this kind of situation, because you can snap the lower object's vertices to the bottom of the upper object.
As I stated in the beginning, there are so many ways to do this, plus I don't know your skill level or the details of what you are aiming to do, so it's hard to know how to answer. But I hope this helps. Feel free to ask questions.